1. Field of the Invention.
This invention relates generally to digital receivers and particularly to apparatus and methods for increasing the bandwidth of a conventional digital receiver to provide improved frequency detection.
2. Description of the Prior Art
Digital processing of a signal such as an electromagnetic wave begins with receiving the wave with an antenna and forming a corresponding analog electrical signal. The electrical signal is then digitized using an analog-to-digital converter (ADC), which samples the wave at a sampling frequency f.sub.s and forms a digital waveform using the magnitude of the analog waveform at each sampling time. The process of digitizing the signal is usually governed by the Nyquist criterion, where it is assumed that the input signal must be band limited such that 0.ltoreq.f.ltoreq.f.sub.s /2 before going into the analog-to-digital converter (ADC). If the sampling frequency is less than twice the incoming signal frequency, then the analog waveform is undersampled. If the signal contains a frequency component higher than half the sampling frequency, aliasing occurs, which causes ambiguities in the analog-to-digital conversion process.
There are several advantages to an undersampled system. Among these are a reduction in the speed requirements on the digital section of the system, a relaxation on the analog antialiasing filter requirements, and the possibility of extending the capabilities of existing systems with relatively minor redesign. In addition, there are power and cost savings in the ADC section.
The main problem in identifying the frequencies present in an undersampled signal is the resolution of the ambiguities. The Nyquist theorem places a limitation only on the information that can be derived from a single set of digitized data. That is, a single set of digitized data limits subsequent analysis to an f.sub.s /2 bandwidth unless there is additional information available. With additional information, the frequency components f&gt;f.sub.s /2, which appear ambiguously due to undersampling, may be resolved. A prior undersampling technique uses a set of trial sampling periods to recover periodic signals by reconstructing the waveform. The trial period that yields the waveform of smallest variation is then considered to be the correct period and the resulting waveform the correct waveform. To come more in line with real-time wideband processing, other methods based on the use of phase shift information to resolve the ambiguities in a single frequency undersampled signal have also been investigated. These procedures are complex and resolve only small ranges of ambiguities.